This invention relates to a piezoelectric transformer circuit which includes a piezoelectric transformer unit.
Heretofore, such a transformer unit has been proposed by C.A. Rosen in Proceedings of Electronic Components Symposium held in Washington, D.C., May 1, 2, 3, 1956 and makes use of a pair of polarized ferroelectric ceramic bars one of which is polarized in a direction of a thickness and the other of which is polarized in a longitudinal direction thereof and both of which are combined with one another. As well known in the art, when the one ceramic bar is driven by an a.c. voltage, the other ceramic bar is excited in a longitudinal vibration mode to induce a high output voltage because both the ceramic bars are electromechanically coupled to each other.
However, the transformer unit is disadvantageous in that a frequency characteristic are degraded frequency band, which inevitably makes the frequency characteristic narrow.
In order to improve the frequency characteristics, Inoue et al disclose a thickness mode vibration piezoelectric transformer unit in U.S. Pat. No. 5,118,982 assigned to the same assignee to the instant application. Such a thickness mode vibration piezoelectric transformer unit will be called a piezoelectric transformer unit hereinafter.
The proposed piezoelectric transformer unit comprises low and high impedance thickness mode vibration portions which are integrally laminated or stacked on each other. The high impedance portion is composed of a thick piezoelectric layer interposed between upper and lower electrodes while the low impedance portion is composed of a plurality of thin piezoelectric layers which are stacked from a lowermost one to an uppermost one and each of which is interposed between two electrodes. One of the electrodes that is attached to the lowermost thin piezoelectric layer is used in common to the upper electrode attached to the thick piezoelectric layer. In addition, the thick and the thin piezoelectric layers have thicknesses different from each other and are polarized in a thickness direction so as to accomplish the thickness mode vibration.
More specifically, polarization of the thick piezoelectric layer is directed in either an upper sense or a lower sense in the thickness direction while polarization of each thin piezoelectric layer is alternately directed in an upper or a lower sense in the thickness direction. In the high impedance portion, the lower electrode on the thick piezoelectric layer is connected to a first external terminal while the upper electrode on the thick piezoelectric layer is connected in common to the electrode attached to the lowermost thin piezoelectric layer and is grounded through a second external terminal. In the low impedance portion, every other electrode of the thin piezoelectric layers is electrically connected in common to a third external terminal while the remaining electrodes of the thin piezoelectric layers are also connected in common to a fourth external terminal which is grounded. The second and the fourth external terminals may be connected in common to each other.
From this fact, it is readily understood that the piezoelectric transformer unit has a three-terminal construction. With this structure, when an a.c. input voltage is given between the first and the second (or the fourth) external terminals, an a.c. output voltage is induced between the second or the fourth external terminal and the third external terminal, as mentioned in the referenced United States Patent. Accordingly, the proposed piezoelectric transformer unit serves to effectively transform the a.c. input voltage into the a.c. output voltage when it is excited in a resonance mode.
Eventually, the piezoelectric transformer unit is small in size and can improve a high frequency characteristic. However, neither suggestion nor consideration is made about application of such a piezoelectric transformer unit to any other circuits in the above-referenced United States Patent.